The overall goal of the Group Project is to find an appropriate replacement for isoniazid and rifampin in the treatment regimens for MDRTB, and for isoniazid in preventive therapy. The objective of Project No. 1 is to discover the most promising representatives of three classes of agents, fluoroquinolones, macrolides, and new rifamycins, by evaluating their antibacterial activity in culture media and in macrophages. The studies will be conducted with drugs singly and in combination, assessing their inhibitory, bactericidal and sterilizing activity under various experimental conditions. The prospective drugs are those recently synthesized by three pharmaceutical companies (Parke- Davis, Abbott, Kaneka-Japan), who have already provided us with 15 new compounds. We anticipate a continuous supply of new drugs from these three companies, but our future studies will not be limited to the drugs obtained from these companies. Minimal inhibitory concentrations (MIC), minimal bactericidal concentrations (MBC) and postantibiotic effect (PAE) will be determined against both extracellular and intracellular bacteria. MIC, MBC and PAE against extracellular bacteria will be determined by the methods summarized in the 1991 monograph written by the PI. The MICs will be determined in liquid medium (7H12 broth) by viable counts, radiometrically, and in agar plates. The MBC, MBC/MIC ratio and PAE will be determined by viable counts in liquid medium. The same values against intracellular bacteria will be determined in two types of macrophage cultures: human monocyte-derived and J774. The studies with macrophages will also include determination of the actual intracellular accumulation of each drug using the velocity gradient centrifugation to separate the cells after they have been exposed to the drug. The potential of drugs singly and in combinations for preventive therapy will be evaluated in macrophages infected with M. tuberculosis after they have been exposed to drugs for a period of two days. Sterilizing activity will be evaluated against semi-dormant broth cultures showing no changes in the viable counts but with evident metabolic activity. The first step of evaluation of each agent will be determination of the MIC and MBC in culture medium. The potential of an agent for future studies will be determined on the basis of comparison of the MIC to the pharmacokinetic parameters provided by the manufacturer and by the value of their bactericidal activity (MBC/MIC ratio). The most promising agent in each class will be the subject of further studies in vitro and in macrophages and will also be evaluated in an animal model under Project No. 2 and in regard to the problems of drug-resistance prevention.